The present invention generally relates to a method for removing unwanted coating material from a semiconductor wafer and more particularly, relates to a method for removing a bead of a coating material of spin-on-glass or photoresist from a wafer flat side by a cleaning solution.
Spin-on-glass (SOG) is frequently used for gap fill and planarization of inter-level dielectrics (ILD) in multi-level metalization structures. It is a very suitable material for use in low-cost fabrication of IC circuits. The most commonly used SOG materials are of two basic types; an inorganic type of silicate based SOG and an organic type of siloxane based SOG. The most commonly used SOG materials are silicon oxide based polysiloxanes. These polysiloxanes are featured with radical groups replacing or attaching to oxygen atoms. Based on these two basic structures, the molecular weight the viscosity and the desirable film properties of SOG can be modified and adjusted to suit the requirement of each specific IC fabrication process.
SOG film is typically applied to a pre-deposited oxide surface as a liquid to fill gaps and steps on the substrate. Similar to the application method for photoresist films, a SOG material can be dispensed onto a wafer and spun with a rotational speed which determines the SOG thickness desired. After the film is evenly applied to the surface of the substrate, it is cured at a temperature of approximately 400xc2x0 C. and then etched back to obtain a smooth surface in preparation for a capping oxide layer on which a second interlevel metal may be patterned. The purpose of the etch-back step is to leave SOG between metal lines but not on top of the metal, while the capping oxide layer is used to seal and protect SOG during further fabrication processes. The siloxane based SOG material is capable of filling 0.15 micron gaps and therefore it can be used in 0.25 micron technology.
When fully cured, silicate SOG has similar properties like those of silicon dioxide. Silicate SOG does not absorb water in significant quantity and is thermally stable. However, one disadvantage of silicate SOG is the large volume shrinkage during curing. As a result, the silicate SOG retains high stress and cracks easily during curing and future handling. The cracking of the SOG layer can cause a serious contamination problem for the fabrication process. The problem can sometimes be avoided by the application of only a thin layer, i.e., 1000xcx9c2000 xc3x85 of the silicate SOG material. To build up thicker layers for filling gaps, multiple application and curing are required.
In the current SOG coating process, a solvent edge rinse and a solvent backside rinse process are utilized to remove SOG deposited on the wafer edge and on the backside of the wafer. This is shown in FIGS. 1xcx9c3. A semiconductor wafer 10 that has a flat side 12 is shown in FIG. 1. After a SOG coating process, a SOG layer 14 is blanket deposited on the top surface 16 of the wafer. The SOG layer is deposited as a dielectric layer for insulating between metal lines. In order to process the wafer in subsequent fabrication steps, the wafer must be positioned in various reaction chambers for various processes such as etching or deposition. In most of the process chambers, the wafer is positioned on a platform and held down on the edge by a wafer clamp. The function of the wafer clamp is to prevent the wafer from moving during the process where reactant gases or etching gases may be flowing through the reaction chamber. To enable the wafer clamp to function properly, the edge portion of the wafer of approximately 2xcx9c4 mm wide must be cleaned and any coating layer removed. The edge area 22 is shown in FIG. 1 on wafer 10.
In present wafer fabrication technology, the SOG layer deposited at unintended areas of the wafer can be removed in two different processes. The first process is a solvent edge rinse which is shown in FIG. 2. In this process, wafer 10 is placed on a platform (not shown) and spun at a predetermined rotational speed along a spin axis 26. The rotational speed of the wafer can be suitably adjusted for each specific application depending on the thickness of the layer to be removed and the type of chemical solution used. As shown in FIG. 2, a chemical solution injector 28 is used to inject chemical solution 32 onto the top edge 34 of the wafer. The chemical solution 38 reflected from the edge 34 of the wafer hits the chamber wall 42 and drain to the bottom of the process chamber. The solvent edge rinse process is effective in removing a limited area, i.e., a width of 2xcx9c4 mm, on the top edge of the wafer of unwanted coating materials such as SOG or photoresist. However, as shown in FIG. 1, when wafer 10 is spun around its center as a rotational axis, the flat side 12 of the wafer is not touched by the injected solvent 32 each time the wafer rotates. As a result, coating material 18 in the form of a bead remains on wafer 10.
The second cleaning process is a solvent backside rinse such as that shown in FIG. 3. The backside 48 of wafer 10 is cleaned in this process. A cleaning solution 52 is injected from a spray nozzle 54 onto the backside 48 of the wafer. The process is also known as a centrifugal spray cleaning process wherein a chemical solution, i.e., normally of a good solvent for the coating layer to be removed, is pressure-fed and injected directly onto a spinning wafer. The process can be effectively used to reduce the volume of fresh chemical consumed and is normally faster than an immersion process. After the injected chemical solution 52 hits the bottom surface 48 of the wafer, the chemical solution 56 reflects from the backside 48 of the wafer and drains into the bottom of the process tank (not shown). During a normal backside rinse process, the sprayed chemical solution 52 is only capable of rinsing on the backside 48 of the wafer and, none of the chemical solution 52 can reach the top surface 16. The bead 18 at the flat side 12 of the wafer is therefore not affected or cleaned in the backside rinse process.
Consequently, the SOG bead remains on the flat side of the wafer and eventually leads to SOG cracking during subsequent processes when a wafer clamp is pressed down on the SOG bead for mounting the wafer. The particles generated by the cracking of the SOG layer greatly contaminate the surface of the wafer and is detrimental to the yield and the quality of the IC produced.
It is therefore an object of the present invention to provide a method for removing a coating layer from an unintended area on a wafer that does not have the drawbacks or shortcomings of the conventional cleaning methods.
It is another object of the present invention to provide a method for removing a coating layer from wafer flat side to prevent the formation of a bead of the coating material.
It is a further object of the present invention to provide a method for removing a coating layer from an unintended area on the wafer flat side such that the wafer can be processed in subsequent processes without producing particle contaminants.
It is another further object of the present invention to provide a method for removing a spin-on-glass material from a wafer flat side such that a SOG build up at the flat side can be avoided.
It is still another object of the present invention to provide a method for removing a SOG layer from wafer flat side by utilizing existing backside rinse equipment.
It is yet another object of the present invention to provide a method for removing a SOG coating layer from wafer flat side by utilizing a predetermined rotational speed of the wafer such that a cleaning solution can be drawn from the underside of the wafer to the top side of the wafer.
It is still another further object of the present invention to provide a method for removing a coating material from wafer flat side by utilizing a combination of a desirable rotational speed and a desirable solvent mixture such that the solvent can be drawn from the bottom to the top of the wafer around the edge of the wafer.
The present invention provides a method for effectively removing a coating material such as SOG or photoresist layer from the wafer flat side such that a build up of the coating material and possible cracking in subsequent processing steps can be prevented.
In a preferred embodiment, the method for removing a coating material from an unintended area on a wafer can be carried out by the steps of first providing a wafer that has a top surface and a bottom surface, the top surface has a coating layer covering an unintended area along the edge of the wafer, spinning the wafer at a predetermined rotational speed, and injecting a flow of a cleaning solution at the bottom surface of the wafer adjacent to the edge of the wafer, whereby the predetermined rotational speed of the wafer pulls the cleaning solution from the bottom surface of the wafer to the top surface of the wafer to clean away the coating layer deposited at the unintended area. The flow of cleaning solution can be injected at the bottom surface of the wafer in an area immediately adjacent to the edge while the wafer is spun at a rotational speed of not less than 150 rpm. The cleaning solution utilized can be a mixture of one or more solvents selected from the group consisting of C5H10O3 and C3H8O at a suitable mixing ratio.